High-frequency, electronic devices are commonly required to operate at frequencies up to 100 GHz during the normal course of operation. For example, cellular phones, wireless-Local-Area-Networks (LANs), digital-radio links, military radar, and many other high frequency applications typically have operations therein using frequencies typically up to 100 GHz. Because such high frequencies are used during operation, integrated circuits (ICs, sometimes referred to as Monolithic Microwave Integrated Circuits or MMICs in high-frequency applications) are typically fabricated from Gallium Arsenide (GaAs) as opposed to silicon as it is well known that GaAs is better suited for high-frequency applications.
It is also well known that GaAs (or other semiconductor materials, such as silicon) can be fabricated in wafers that are optimal for enhancement mode operation or depletion mode operation. Enhancement mode and depletion mode are realized by the nature of the fabrication process and each offers advantages over the other depending upon the application required by wafer-level components. For example, a wafer fabricated to be optimal for depletion mode is better suited for components such as switches, low-noise amplifiers, and DC current sources. On the other hand, a wafer fabricated to be optimal for enhancement mode is better suited for components such as linear or power amplifiers, simple digital logic, and DC current mirrors. Thus, because each mode provides certain advantages, it is desirable to have each mode available for operations in an electronic device. Operating an electronic device with both modes available is often called enhancement/depletion (E/D) mode operation. Fabricating wafers separately as either enhancement mode or depletion mode is well-known in the industry.
In the past, ICs were fabricated (in wafer form) to be optimal for performance in either enhancement or depletion mode. In more recent advances, some ICs were fabricated with two regions, where a first region was fabricated to be optimal for enhancement mode and the other region was fabricated to be optimal for depletion mode. However, because of the nature of the etching process involved with fabricating the wafer, it proved to be difficult to fabricate ICs from GaAs for optimal use in both modes. As a result, a typical IC fabricated for use in devices requiring E/D mode would sacrifice optimal performance in the least-application-significant mode. That is, a designer would choose optimal performance in depletion mode at the expense of optimal performance in enhancement mode or vice versa. In high-frequency applications, optimal performance in both modes becomes more important than in lower-frequency applications.
As advances were made in etching and masking, wafers having one area optimized for enhancement mode and another area optimized for depletion mode were able to be fabricated. Using a single substrate of GaAs, a complicated etching and masking process can be accomplished, albeit expensively and laboriously. This is because GaAs epitaxial wafers have a very complex material growth stack (layers) dedicated to only MESFET components and the like for use in depletion or enhancement mode. To grow an E/D mode wafer, the epitaxial material must be grown as two complete stacks (one for enhancement mode, one for depletion mode). The fabrication process is further complicated with separation layers that provide various etch stop (or another barrier technique) to isolate the enhancement region from the depletion region. Often, such epitaxial material is very difficult to work with and expensive such that compromises must be made at the cost of performance or isolation of the regions. The fabrication process is still further complicated by complex etching techniques that must be employed to select the buried material in one region or the other for the particular transistors needed. Thus, fabricating a single wafer to have two different regions for enhancement mode and depletion mode is expensive, time-consuming, and difficult.
Another solution has been to use two different ICs in an electronic device; one for enhancement mode and one for depletion mode. This solution, however, requires two separate ICs and additional off-chip wire bonding such that the benefit of the high frequencies available using GaAs is lost in the wire bonding.
As such, it is desirable to have an easily-fabricated, wafer-level, MMIC capable of providing optimal performance in both enhancement mode and depletion mode and fabricated from GaAs or another semiconductor suitable for use in high-frequency applications.